1. Field of the Invention
The present invention relates to a wavelength-division multiplexing optical access network, and more particularly, to an optical transmission apparatus and an optical access network for a wavelength-division multiplexing optical network with both sub-carrier multiplex and sub-carrier multiple access schemes.
2. Description of the Related Art
Digital subscriber line (xDSL) technologies have been developed to provide data services to subscribers using existing telephone lines. Also, data service schemes have been suggested to provide data services through cable networks using coaxial cables.
Such existing data service technologies do not have problems in terms of supporting telephone traffic and Internet traffic. If ultrahigh speed services such as remote video conferences, high definition television (HDTV) services, remote education, remote diagnoses and treatments are generally available to subscribers, the existing data service technologies are limited by bandwidth and distance, and thus are not able to provide sufficient wideband, high quality services.
Optical access network technologies have been suggested in order to provide wideband, high quality services to users. In particular, wavelength-division multiplexing optical networks have been recently studied as optical access network technologies.
The wavelength-division multiplexing optical access networks are attractive as next generation access networks since the wavelength-division multiplexing optical access networks can provide a large amount of information to subscribers, and are highly secure.
However, the wavelength-division multiplexing optical access networks are costly in terms of installation and maintenance.
Different from Back-Bone networks having a small number of channels and high transmission speeds, in general the access networks require relatively low transmission speeds and include a large number of channels. Thus, new networks are required to be capable of accommodating a large number of subscribers using a small amount of communication resources, i.e., maximizing the number of subscribers per a single optical fiber, which as a result solves the cost reduction issue.
The schemes, each wavelength optical channel of wavelength division multiplexing optical access network joined with TDM or TDMA, sub-carrier multiplex and sub-carrier multiple access networks using a large number of sub-carriers to accommodate a large number of subscribers are considered as alternative options.
The advantages and disadvantages of time-division multiplex and time-division multiple access schemes and sub-carrier multiplex and sub-carrier multiple access schemes will now be described in more detail.
In the time-division multiplex and time-division multiple access schemes, data of a large number of subscribers is statistically multiplexed in order to efficiently use resources.
However, accurate synchronization is required, and the states and requests from subscribers affect the access of other subscribers, and packet overheads increase due to periodical polling and ranging.
In the sub-carrier multiplex and sub-carrier multiple access schemes, different sub-carriers are allocated to subscribers sharing an optical fiber, the information are transmitted through the allocated sub-carriers to each of subscribers, and at the receivers the information are filtered through sub-carrier band pass filters assigned to each of the subscribers.
Optical access networks using sub-carrier multiple access schemes do not suffer from the increase in overhead as in those using time division multiple access caused by timing synchronization, periodical polling, and ranging. However, optical beat interferences (OBIs) may occur in signal bands when receivers in the central base stations simultaneously receive a plurality of light sources. As a result, the OBIs lower a signal-to-noise ratio (SNR).